1) Field of the Invention
The invention relates to laser beam shaping. More particularly, the invention relates to a beam shaping system for providing a square or rectangular laser beam with controlled intensity distribution.
2) Description of the Prior Art
While most laser sources and more precisely laser diodes sources produce an astigmatic beam of light having a substantially non-uniform intensity profile, numerous laser applications require a uniform illumination of a rectangular target. Such applications include biomedical applications, such as bio-detection, wherein, for example, a uniform illumination of a blood sample is required. Other applications include micromachining, microscopy, night vision and range finding of distant object.
Shaping a Gaussian-like laser beam using diffractive optics can provide a flat-top laser beam. One drawback of diffractive beam shapers is the wavelength dependency of their optical response. Another drawback is the low efficiency. Diffractive beam shapers are thus not suitable for wide spectrum or multiple wavelength illumination.
Refractive beam shaping techniques are efficient and provide low wavelength dependency. Conventional refractive techniques using aspherical lenses are suitable for generating a rotationally symmetrical flat-top beam from a rotationally symmetrical Gaussian input beam, but they are not adapted to shape an incident beam that is not rotationally symmetrical, like laser diode beams. Laser diodes have an elliptical intensity profile and suffer from astigmatism.
U.S. Pat. No. 4,826,299 to Powell, provides a lens for expanding a laser beam along one axis in order to provide a laser line of uniform intensity and width. Such a diverging lens has an acylindrical surface defined by a base curve in the shape of an angle with a rounded apex. The radius of curvature of the acylindrical surface is thus smaller in the center and increases smoothly towards both ends. As described in Powell, the acylindrical surface fits to a base curve defined in a Cartesian coordinate system (x,y,z) by the following equation:
  y  =            cx      2              1      +                        (                      1            -                                          (                                  1                  +                  Q                                )                            ⁢                              c                2                            ⁢                              x                2                                              )                          1          ⁢                      /                    ⁢          2                    wherein c is a curvature constant and Q is a conic constant, and wherein the product Q.c lies between 0.25 and 50 mm−1 and Q is less than −1. The second surface of the acylindrical lens may either be flat or cylindrical.
Acylindrical lenses have been created and used in the prior art for providing a laser line of uniform intensity. Laser lines are used, for example, for alignment purposes. The provided laser line should then be long and thin. Acylindrical lenses described in Powell provides a high divergence to provide the required line length.